1. Field of Invention
This invention relates to a process for forming toner particles, toner particles prepared by the process, and developer compositions including the toner particles prepared by the process. More specifically, this invention relates to an extrusion process for the preparation of toner compositions.
2. Description of Related Art
In the process of electrophotographic printing, a photoreceptor containing a photoconductive insulating layer on a conductive layer is imaged by uniformly electrostatically charging the surface. The photoreceptor is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer, while an electrostatic latent image is formed on the non-illuminated areas. Toner particles are attracted from carrier granules to the latent image to develop the latent image. The toner image is then transferred from the photoconductive surface to a sheet and fused onto the sheet.
Various toner compositions for such a printing system are well known in the art and have been produced having a wide array of additives and constituent materials. Generally, toner particles include a binding material, such as a resin, and any of various additives to provide particular properties to the toner particles.
Numerous devices and processes are known for preparing toner particles. Examples of commercially known processes include the melt-blending of toner components in a Banbury roll mill apparatus, spray drying, dispersion polymerization, solution polymerization, and the like. It is well known that toner compositions can be obtained with an extrusion apparatus. An extrusion apparatus and process enables a number of advantages that cannot be achieved with a Banbury roll mill apparatus and process. For example, extrusion is a continuous process, rather than a batch process, and extrusion processes can be easily automated, allowing for more economical toner preparation.
Certain additives useful in toner particles are partially or totally immiscible in the resin materials with which they may be used. Although this immiscibility is advantageous for some additives, such as waxes, which must remain in a distinct phase in order to properly function, the toner itself must be relatively homogeneous to function properly. Thus, it is necessary to disperse the immiscible components in the blending process. Two known methods for homogenously dispersing additives are the use of a compatibilizer and the injection of a liquid into the blending chamber during the extrusion process.
The use of a compatibilizer to disperse additive particles, in particular wax particles, has drawbacks for toner production. For example, it is known that the grinding rate of the extruded material is reduced by compatibilizers that have been incorporated into the resin materials to disperse additives. The reduction of the grinding rate increases the time that the composition must remain in residence in the grinder and often results in the grinding step limiting the overall production rate. This increased residence time increases the total production time and costs, and decreases the efficiency of the production process.
Water or liquid injection, which lowers the melting temperature to disperse additive particles in toner resin compositions, is an alternative to the use of a compatibilizer. The bulk of the injected liquid, along with any volatile materials, is removed by the application of a vacuum prior to the discharge of the toner formulation from the toner processing apparatus. However, vacuum equipment is difficult and expensive to operate and maintain.
U.S. Pat. No. 4,973,439, which is incorporated herein in its entirety, discloses a toner processing apparatus for obtaining toner particles with improved dispersion of additive components, comprising a blending chamber, a mixing screw, a heater, a supply means, an injector for injecting liquids enabling a decrease in the melting temperature of the resin particles, and a vacuum extractor for removing the injected liquid prior to extrusion.
There remains a need for an alternative to the above methods for dispersing additives throughout the resin in toner particles. In particular, there is a need for a method of dispersing additives without reducing the grinding rate, as occurs with the use of compatiblilizers, and without the need for expensive vacuum equipment.